The invention relates to a proximity switch operating in a non-contacting manner and comprising a tank circuit influenceable by objects approaching it from the outside, evaluating stages for generating an output signal as a function of an output quantity of the tank circuit exceeding a predetermined threshold value, terminals for applying a supply voltage and at least one further terminal for output of the output signal and programming trimming devices for adjustment of a trimmable circuit parameter.
Such a proximity switch, more precisely, an inductive proximity switch, is known from German published patent application 36 08 639. In the known proximity switch, the resistivity of a resistor connected in parallel with an oscillator circuit is adjusted when a predetermined distance between an object and the active surface of the sensor part of the proximity switch is reached by monitoring the damping of the oscillation of the tank circuit when several resistors in parallel connection are deactivated one after the other by breaking a switch connected in series with the respective resistor. The thus determined combination of one or several resistors is then permanently and irreversibly fixed by blowing fuses.
The known proximity switch has the disadvantage that the trimming of the resistivity can only be carried out once and that errors in this adjustment can never be corrected again.
A particularly serious disadvantage of the known switch according to German published patent application 36 08 639 is that the proximity switch has to have a special additional terminal for feeding the proximity switch a control signal when trimming of the resistivity is to be carried out.
It is, furthermore, known from U.S. Pat. No. 4,502,042 to alter the sensitivity or reaction distance in a proximity switch by adding different resistors. However, in the known proximity switch, this measure is not for trimming purposes but for determining the position of an object to be detected within a maximum detectable measuring range after adjustment of the oscillator has already been carried out. The reaction distance is traversed cyclically from larger to smaller values or vice versa by connecting the resistors without enabling a precisely predetermined reaction distance to be reached by a simple connection to a predetermined combination of resistors.